Heating and cooling of the building sector accounts for 40% of total energy consumption in large economies such as United States of America (USA), Canada, China, India, and European Union. The USA, where almost 90% of homes have air conditioning, uses as much electricity to keep buildings cool as the total electricity used in the entire continent of Africa. Although air conditioning is still relatively uncommon in low-income, developing countries, this is poised to change dramatically. Worldwide power consumption for air conditioning alone is forecast to surge 33-fold by 2100 as income levels of the developing world rise and urbanization advances. By the mid-century, the world will use more energy for cooling than heating. The growth is not driven by high-income countries like USA, instead by middle-income countries where households and businesses are buying air conditioners at astonishing rates.
Apart from air-conditioning to enhance comfortable living, other cooling requirements have become a significant part of twenty-first century life in advanced economies: most food in the developed world is chilled or frozen; medicines, including vaccines, need refrigeration; industries such as steel, chemicals, petroleum, and plastics depend on cooling; social media networks and data centers, which power the internet and other information resources, require cooling systems to run continuously and reliably to prevent a system collapse.
However, meeting the increased demand for electricity will be an enormous challenge. Trillions of dollars of investments will be required in electricity generation and transmission infrastructure. In addition, most electricity worldwide continues to be generated using fossil fuels, so this growth in air conditioning would result in striking increases in carbon dioxide emissions. The generation of power accounts for the majority of greenhouse gas emissions and associated climate change, which is the most serious challenge currently faced by all earth's inhabitants. It is predicted that cooling the built-environment makes the planet hotter. These challenges emphasize the necessity of developing greener, sustainable cooling systems that minimize the negative human impacts on the natural surroundings, materials, resources, and processes that prevail in nature.
Heat-driven air conditioning systems and heat-pump systems use low-grade thermal energy like solar energy and waste heat rather than electricity. These systems can produce significant energy savings, have low global warming and ozone depletion potential, and ensure higher indoor air quality. Studies have demonstrated that such heating/cooling equipment is applicable to wide ranges of climate conditions and offers a feasible alternative to conventional heat-pump systems driven by electricity. In the present disclosure, an innovative heat-driven refrigerator/heat-pump system, which comprises of a single-effect absorption unit that operates in conjunction with a combination of expanders and compressors is presented. In this section, the expander-driven vapor compression refrigeration device and the heat-driven vapor absorption refrigeration device are described briefly as they form the basis for the illustrative embodiments. Further, since some of the illustrative embodiments presented in this disclosure utilize ejector devices, a brief description of the working principles of the ejector devices will also be included in this section.
Vapor Expansion Power Cycle Driven Vapor Compression Refrigeration System (VEPC-VCRS)
The heart of this system comprises of an expander and a compressor which are mechanically linked through their shafts so that the power consumption of the compressor is fully met by the power output of the expander while a common working fluid is used in both devices. The expander is fed with hot vapor of the working fluid from a suitable vapor-producing device such as a boiler where the high-pressure working fluid in the liquid form is vaporized. An optional superheater may be used to further increase the temperature of the vapor generated by the boiler. The compressor extracts the low-pressure vapor of the working fluid from the evaporator where a cold-effect is produced by allowing the working fluid in the liquid form to vaporize at a low pressure and a low temperature. Both power units, the expander and the compressor deliver their outlet vapor streams to a common condenser where the vapor condenses rejecting heat to the condenser cooling medium. The condensate collected in the condenser is split into two streams, of which one stream is pumped to the high-pressure boiler using a mechanical pump, while the other stream is allowed to flash into the evaporator through a throttle valve.
This system can be operated with a variety of working fluids such as common refrigerants, organic fluids, natural refrigerants including water, which could be single pure fluid or mixture of multiple fluids. The coefficient of performance (COP), which is the cold-effect (in terms of heat) produced per unit heat supplied to the boiler/superheater, depends on the operating temperatures of the boiler, evaporator, and the condenser, and the working fluid used in the system. For example, a system which runs on water as the working fluid, the COP is in the range 0.5˜0.7, when the boiler temperature range is 150˜200° C., the evaporator temperature range is 5˜10° C., and the condenser temperature range is 30˜40° C.
Vapor Absorption Refrigeration System (VARS)
The operating principles of Vapor Absorption Refrigeration System (VARS), also known as the Single-effect Absorption Refrigeration System compared to Vapor Compression Refrigeration Cycle (VCRC) in one sense very similar, as they both use a condenser to condense the refrigerant vapor, a throttle valve to expand and flash the refrigerant condensate into the evaporator, and the evaporator to evaporate liquid refrigerant at a low pressure producing a cold effect. However, they also differ in the sense as to how the vapor is compressed from the evaporator to the condenser. The VARS makes use of the principle that pumping a liquid between two pressures does not cost as much mechanical power as compressing a vapor between the same two pressures. Thus, in VARS a solution consists of the refrigerant and an absorbent is circulated using a mechanical pump between the low-pressure absorber and the high-pressure generator.
The generator of VARS differs in operation to a typical boiler used in VEPC-VCRS. In a boiler, the working fluid enters as a liquid stream, completely vaporizes due to boiling, and exits as a vapor stream. Whereas in the generator of VARS the refrigerant vapor is generated by boiling a mixture of refrigerant/absorbent rich in refrigerant, commonly known as the solution. As a result, a strong solution (rich in refrigerant) enters the generator, a portion of the refrigerant is vaporized from the solution due to boiling, the refrigerant vapor is expelled from the vapor outlet of the generator, and the weak solution (weak in refrigerant) exits from the solution outlet of the generator.
The refrigerant vapor which is generated in the generator condenses in the condenser. The resulting condensate flashes into the low-pressure evaporator through a throttle valve. The liquid refrigerant boils in the evaporator at a low pressure and at the refrigeration temperature, hence produces a cold-effect in the evaporator. The low-pressure vapor generated in the evaporator flows to the absorber in which the vapor is absorbed to the weak solution which flows into the absorber from the generator. In VARS the condenser and the generator operate at the same high pressure, while the absorber and the evaporator operate at the same low pressure. Despite the operating pressure difference of the condenser and the absorber, they both operate at temperatures just above the ambient temperature, as they both reject heat to the ambience. The generator is heated by an external heat source to maintain its high temperature, while the evaporator absorbs heat from the space or the body in which the cold effect to be made.
Apart from the solution circulation pump which was mentioned before, the system also consists of a solution heat exchanger, and a solution blowdown valve which also is known as the throttle valve. The solution circulation pump feeds the strong solution (rich in refrigerant) from the low-pressure absorber to the high-pressure generator and the solution heat exchanger is used to transfer heat from the hot weak solution (weak in refrigerant) which leaves the generator to the strong solution delivered by the solution pump, hence regenerating otherwise wasted heat back to the solution. The solution blowdown valve is placed downstream of the solution heat exchanger, in the absorber solution feedline through which the weak solution flows from the generator to the absorber.
The major constituents of the solution are the refrigerant and a suitable absorbent, while the other reagents may be added to enhance its thermal stability, to improve the flow and heat transfer characteristics, as well as to inhibit corrosion especially in the generator where the corrosion will be the most severe due to its high operating temperature.
The absorption process which takes place in the absorber manifests the following effects.                The weak solution absorbs the refrigerant vapor and the solution becomes enriched in refrigerant. Therefore, the solution enters the absorber as weak solution, and it leaves the absorber as strong solution.        Since the absorption phenomenon is an exothermic process, the heat is rejected to the cooling medium of the absorber.        The absorber cooling which sustains and enhances the absorption process is what causes the low pressure in the absorber. Therefore, to a high degree, the heat transfer performance of the absorber determines the performance of the absorber, hence the overall performance of the system.        
The stable operation of the system is sustained on one hand by the generator which continually supplies the high-pressure refrigerant vapor to the condenser, thus providing a continuous stream of refrigerant condensate to the evaporator, on the other hand by the absorption process in the absorber which maintains a low pressure in the absorber and hence in the evaporator which is directly connected to the absorber, causing the refrigerant to boil at a low refrigeration temperature in the evaporator.
The generator and absorber pressures can be varied depending on:                The upper bound operating temperature of the generator and the lower bound operating temperature of the absorber        The relative operating solution concentration range in the generator/absorber unit        The way in which the vapor is conveyed from the evaporator to the absorber and/or from the generator to the condenser.        
In the current state of the art, the ammonia (NH3)-water and water-Lithium Bromide (LiBr) VAR systems are the extensively used where low grade or waste heat is cheaply available. A single-effect water-LiBr system, where the refrigerant is water, can be driven with 80˜95° C. hot water or low-pressure steam, can provide a marginally better performance (COP in the range 0.7˜0.8), although a lower-bound evaporator temperature exists around 5° C. Thus water-LiBr system finds its applications limited to air-conditioning and other sub-ambient temperature chilling operations such as wine/beverage cooling.
On the other hand, the NH3-water VARS requires high temperature heating medium such as high-pressure water or steam at 100˜150° C. and performs marginally lower (COP in the range 0.5˜0.7), however a sub-zero temperature cooling effect, as low as −10˜20° C. can be achieved.
One advantage of the water being the refrigerant is that it has a high latent heat content. This enables one to modify the water-LiBr absorption chiller to exploit a second low-pressure generator which will be driven from the high latent heat content of the water vapor generated from a high-pressure generator, leading to the double-effect chiller design. In the double-effect water-LiBr chiller, external heating medium (in the range 100˜150° C.) drives a high-pressure generator producing high-pressure water vapor, by condensing which in turn provides the heat to drive a low-pressure generator. The double-effect water-LiBr VARS performs far superior to the single-effect VARS providing a COP in the range 1.1˜1.3. In the current state of the art one finds the extension of the double-effect strategy to realize a triple-effect VARS which gives a COP in the range 1.3˜1.5 at a driving heat medium temperature in the range 160˜200° C.
Despite the high attainable COP values, double- and triple-effect water-LiBr systems suffer from one major practical drawback which makes these systems costly to maintain, namely the severe corrosion issues experienced, especially in the high-pressure/high-temperature generator. The highly corrosive LiBr salt solution not only decays the heating tubes (mostly copper) and the generator container walls (mostly steel), the galvanic corrosion reaction mechanisms between the salt solution, and copper/steel metal substrates lead to the generation of non-condensable gasses inside the generator which eventually collect in the condenser. The collection of non-condensable gasses in the condenser eventually deteriorates the system performance unless there is a means to extract them from the condenser.
Ejector Devices
The aim of this subsection is to briefly describe the working of an ejector device in the context of its usage in some of the illustrative embodiments presented in this disclosure. An ejector device can be considered as a pumping device or a low pressure creating and maintaining device. In its basic form, an ejector has a relatively simple construction with three fluid ports with no moving parts. With reference to FIG. 1(b), the inlet ports are Primary Inlet Port 60a and Secondary Inlet Port 60b, while the outlet port is the Discharge Port 60c. The main function of the device is to extract and compress the low-energy Secondary fluid which enters through the Secondary Inlet Port 60b, using the high-energy and high-momentum of the Primary fluid which enters through the Primary Inlet Port 60a, by allowing them to mix in the Mixing Chamber 64 and then increase the mixed stream's pressure by converting its kinetic energy to pressure energy as it flows through the Diffuser section 66 of the ejector. Finally, the high-pressure, low kinetic energy mixed stream exits the device through the Discharge Port 60c. 
Its physics can be described as follows with reference to FIG. 1(b). High-pressure motive fluid stream enters from the Primary Port 60a into the primary nozzle 61. The motive fluid expands through the primary nozzle 61 gradually decreasing its pressure and accelerates increasing its velocity, hence increasing the kinetic energy and the momentum of the primary stream 62. The motive fluid stream 62 leaves the primary nozzle exit at a state of high kinetic energy and momentum, and in the mixing-chamber 64 meets the low-pressure secondary fluid stream 63 which enters the ejector through the Secondary port 60b. In the mixing chamber 64 both fluid streams mix and flow unidirectionally, while the secondary stream 63 accelerates and the primary motive stream 62 decelerates due to the exchange of kinetic energy and momentum between the two streams. Depending on the ejector mixing chamber design, the mixed stream enters the throat 65 with a reasonably high kinetic energy and then enters the diffuser section 66 of the flow passage. In the diffuser the mixed stream decelerates increasing the stream pressure and eventually leaves the diffuser at an intermediate pressure compared to the high motive pressure and the low suction pressure.
Depending on the physical state of the primary fluid at the entrance to the primary nozzle 60a whether it is a liquid or vapor and whether the accelerated stream at the exit of the primary nozzle 61 is in the subsonic or the supersonic, the primary nozzle could be a convergent type or a convergent-divergent type nozzle. For example, if the primary fluid is a liquid, since liquids never accelerates beyond subsonic regime, the primary nozzle is always a convergent type for a liquid. However, if the primary stream fluid physical state is vapor, since a vapor can accelerate to subsonic or supersonic regime, the primary nozzle could be either a convergent type or a convergent-divergent type.
The ejector is said to be a two-phase ejector, when either the primary stream is a liquid and the secondary stream is a vapor or vice-versa. In a two-phase ejector the fluid flow characteristics inside the ejector markedly differ depending on whether the vapor condenses inside the ejector.
The ejector can be designed to pump a given secondary stream of fluid at given rate using a high-pressure primary (motive) stream of fluid. It also can be designed to maintain a given-low pressure in the chamber (not shown in FIG. 1(b)) which is connected to the secondary port.